1. Technical Field
The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine and a multifunctional machine, and in particular, to an image forming apparatus capable of forming an appropriate image in accordance with a position of a transfer sheet in the main scanning direction.
2. Description of Related Art
In the image forming apparatus, printing is carried out in the way wherein an image is formed on a photosensitive drum in an image forming section based on image data obtained through reading with an original reading device, and the image is transferred onto a transfer sheet which has been conveyed from a sheet supply section, and is further fixed.
In the aforesaid printing, if the transfer sheet is conveyed at the same position constantly, the same image can be formed in the transfer sheet, in the image forming section. However, there are some cases where some slight positional slips of the transfer sheet are generated by various types of slips and skews of the transfer sheet generated in the conveyance path from the sheet supply section to the photosensitive drum. When the slips and skews of the sheet of this kind are generated, positional adjustment of the sheet in the conveyance direction (sub-scanning direction) can be carried out by the registration roller immediately before the transfer. However, for sheet slips and sheet skews in the direction perpendicular to the conveyance direction, namely, in the main scanning direction, it is necessary to detect positional slips and thereby to change a scanning position in the image forming section, because the positional adjustment is difficult.
To comply with the aforesaid requirements, a sensor to detect an edge position of the transfer sheet has been provided to detect the edge position of the transfer sheet in transit, and a position of writing on the photosensitive drum in the image forming section has been corrected based on the results of the detection, for appropriated image forming, as disclosed in, for example, Unexamined Japanese Patent Application Publication No. 2000-335010, and Unexamined Japanese Patent Application Publication No. 2003-223088.
A partial structure of the image forming apparatus, which makes the aforesaid correction of the writing position possible, will be explained based on FIGS. 11(a)-11(b).
Leading edge detecting sensor 190 is provided at the downstream side of registration roller 185 provided on a conveyance path for transfer sheet 100, and edge detecting sensor 220 that is composed of a close contact type line sensor in the direction perpendicular to the conveyance direction is arranged between the leading edge detecting sensor 190 and the registration roller 185. The edge detecting sensor 220 is provided with light-emitting section 220a composed of a light source for a single color (preferably, a red light source having excellent reactivity), light guide member 221 that receives light coming from the light-emitting section 220a and emits light to the transfer sheet, and with light-receiving section 220b that receives reflected light coming from transfer sheet 100 or from conveyance guide plate 186 positioned outside the transfer sheet.
While emitting light from the light-emitting section 220a corresponding to an operation of registration roller 185, after leading edge detection of transfer sheet by the leading edge detecting sensor 190, the edge detecting sensor 220 receives, with the light-receiving section 220b, light which is reflected strongly from the transfer sheet 100 and light which is reflected weakly from conveyance guide plate 186 that is made black to suppress light reflection. Sensor outputs obtained by the light-receiving section 220b are compared with a threshold value established in advance, and an edge position of the transfer sheet is detected by discriminating an area outside the transfer sheet where only sensor output lower than the threshold value is obtained. Thus, an image is formed at an appropriate position of the photosensitive drum, after correcting a position of an image to be transferred onto the transfer sheet in the main scanning direction by adjusting it according to the aforesaid detected position. Owing to this, an image is formed and transferred at an appropriate position constantly, despite a positional slip of the transfer sheet in the main scanning direction. In the meantime, it is necessary to receive reflected light having sufficient quantity of light from the transfer sheet, to improve the precision of the aforesaid discrimination.
To achieve the foregoing, it is effective to increase an amount of light emission at the light-emitting section. However, in that case, an amount of reflected light coming from the conveyance guide plate is also increased, and the precision of the discrimination is rather lowered. Therefore, there is generally employed a method to improve the discrimination control through appropriate control of an amount of emitted light by controlling light emitting period for the light-emitting section.
Incidentally, in a market of the shortrun printing, a wide variety of transfer sheets are used, and demands of printing for colored sheets are high. However, if the edge position of the transfer sheet is detected by the aforesaid edge detection sensor, the reflectance varies greatly depending on a color of the transfer sheet, and a sufficient amount of reflected light exceeding the threshold value can not be obtained depending on the color of a sheet, in some cases. For this problem, it is considered to establish the threshold value to be low, in which, however, an error grows greater to create an occasion where an edge position cannot be detected accurately.
The present invention has been achieved under the background of the aforesaid circumstances, and its objective is to provide an image forming apparatus wherein excellent edge detecting accuracy can be obtained, without depending on the color of the transfer sheet.